Designing a beta engine

[space_geek]

Hello everyone,

I took a mechanical engineering course over the summer for an elective and learned how to use solid works. I've become interested in stirling engines and would like to design one, but am having a hard time on finding specific relationships and ratios on the internet for a beta type engine.

I understand the basic principles that drive the engine, and the displacer piston being 90 degrees ahead of the power piston, but I'm unsure of other dimensions.

I would like to have just a small beta engine for my desk similar to this one: http://i20.ebayimg.com/08/i/001/30/7d/ee38_35.JPG

• How much room between the displacer piston and the cylinder is needed to allow airflow?
• How long should the strokes be? The diameter of the bore?
• How long should both the power piston and displacer piston be?
• Is there any lubrication needed for the power piston since it needs to seal the air?

I guess a better question would be is there any source online that answers these questions? Or are these more eye-balled than calculated?

Any guidance would be much appreciated, thank you!

[Hawke]

Ive only begun on making Stirlings myself, but there is no magic forumla that fits all applications and types of engines, however something for you you look at until the more advanced folks chime in. Seems you are looking at designing an alpha style stirling. You might consider building some models in scrap materials first to understand the relationships of friction, leaks, and such. and doing LTD(Low Temp Difference) or Gamma designs. I felt like doing a more complicated one at first and then kept getting simpler and simpler until I got one to work finally. Sticking to proven designs first would be my best suggestion I think. However....

Excerpts from my personal RULES OF THUMB notes(most from threads on this forum):
My Wording:
1. Keep the dead volume zones as low as possible
2. Maximize heat source
3. Maximize Cooling
4. Use a regenerator
5. Reduce friction and air leaks

For an ALPHA motor the ratio between the hot and cold cylinders is near enough to 1:1, displacer type motors are 1.5:1.
For a LTD motor 10:1, and up to, or maybe beyond 20:1 is OK.
...short stroke, high revs, lower torque, long stroke, lower revs, higher torque Ian SC

The following was posted:

1. The length of the displacer chamber = 3 times is diameter.
2. The length of the heated chamber = 2/3 of the length of the displacer chamber (cylinder).
3. The length of the cooling chamber = 1/3 of the length of the displacer cylinder.
4. Swept volume of the displacer = 1 1/2 times the swept volume of the power piston.
5. Length of the displacer = 2/3 of the length of the displacer cylinder.
6. Stroke of the displacer = 1/3 of the length of the displacer cylinder.

[space_geek]

Isn't it a beta engine that I'm interested in? Because both of the pistons occupy the same cylinder?

Do those same same 6 points apply to beta engines as well though?

[Ian S C]

space geek, yes its a BETA motor you are planning. There many ways of building hot air engines, ranging from, a number of designs made from tin cans, or there is one on the net, that you can make directly of your printer, its made of paper/light card, to motors machined from steel, aluminium, etc. So you may need tools ranging from a craft knife, to a lathe, and milling machine, and maybe a TIG welder.
For a small engine, made from scrap, have a look in my gallery, you'll find a little beam engine, its a GAMMA motor, and with a 3/8" bore, probably a bit small. Then there is a 2.5 cc GAMMA motor, that can be built as a BETA type, it is from the "Model Engineer" magazine, where it is shown in both types. Think the bore of the power piston is 5/8".
Good luck, keep us informed, and ask questions. Ian S C

[space_geek]

Thanks for the responses so far. I am still wondering if there are certain dimensions I need that are required to get the engine to function? Or can I just start designing the engine how I want it to look, as long as piston sizes etc are similar to other beta engines that already work?

Hawke posted those 6 rule of thumbs, but he had mentioned an alpha engine before. Do they still apply to a beta engine?

[Ian S C]

For what you need to know, don't worry about the ALPHA motor reference, it's a bit different. Hawke's list is the main rules to follow. A good start is the power cylinder, and crankshaft. Settle on the diameter of the piston, and make the stroke about the same, or a little less, ie., a 1" dia piston would be good with a 3/4" stroke, but could also work quite well with a stroke of 1 1/4", short stroke high revs, lower torque, long stroke low revs, higher torque. For a BETA motor with a 1" stroke for the power piston, the displacer would have a stroke of 1 1/2" stroke. Ian S C

[space_geek]

Ok great thanks for the help! I'll start there and see how far I can get.